The present invention pertains to linear amplification of RF signals, for example linear amplification of RF signals using a multicarrier amplifier.
Amplifiers are typically employed to amplify RF signals in order to provide, e.g., increased power for transmission purposes, particularly transmission over an air interface to a receiver such as (for example) a mobile station (e.g., a user equipment unit (UE) such as a cell phone). But in amplifying an input RF signal, the amplifier may add unwanted components due to non-linear characteristics of the amplifier. Such is particularly true when the type of amplifier utilized is chosen for its power efficiency and/or when plural continuous wave RF input signals are applied to the amplifier. Rather than just producing amplified signals corresponding input signals, such amplifier may also output certain additional signals related to the frequencies of the input signals. In this regard, mathematically the output of the amplifier can be expressed as a DC term; a fundamental term (which includes nominal gain for the input signals and an amplitude distortion); and (typically second and third) harmonics terms. The DC term and harmonics can usually be filtered out rather easily, leaving a passband.
The distortion within the passband is not easily removed, but rather is minimized by designing the overall amplifier system in order to compensate for the non-linear characteristics of the amplifier component per se. Such xe2x80x9clinearizationxe2x80x9d of an amplifier system is important in order to avoid distorted signal trajectories and to avoid errors in determining the logic level of individual digital signals.
There are many techniques that can be used to linearize amplifiers. Among the linearization techniques are the following: Back off (in the case of Class A amplifiers); Feedforward; Vector summation; Predistortion, and Feedback. Several of these linearization techniques are briefly described in U.S. Pat. No. 6,075,411 to Briffa et al., which is incorporated herein by reference in its entirety. See also, in this regard, Briffa, Mark, xe2x80x9cLinearisation of RF Power Amplifiers,xe2x80x9d, 1996.
The feedforward technique is advantageous for broadband linear RF amplifier systems. As mentioned briefly above, since the multicarrier input signal is distorted by the non-linearities in the main amplifier, certain intermodulation (IM) products appear at the output. In essence, the feedforward technique generates an error signal by comparing the input signal with the main amplifier output. The error signal is subtracted from the main amplifier output, leaving a (nearly) distortion-free amplified signal.
FIG. 1 illustrates a simplified, example amplifier system 20 which employs a feedforward technique to minimize distortion. The amplifier system 20 comprises a phase and gain adjuster 22 which receives, via coupler 24, an input signal. Output from the phase and gain adjuster 22 is applied to main power amplifier 26. Output from main power amplifier 26 is applied to a coupler 28, and from one leg of coupler 28 via attenuator 30 to subtractor 32. Both subtractor 32 and first loop controller 34 receive, via delay 36, the input signal as obtained from coupler 24. Output from subtractor 32 is applied both to first loop controller 34 and to a second gain and phase adjuster 40. Output from gain and phase adjuster 40 is applied to auxiliary amplifier 42, whose amplified output is coupled by coupler 44 to line 46. Line 46 emanates from coupler 28 and delay 48. The output signal carried on line 46 at point 51 is applied via coupler 50 and attenuator 52 to third loop controller 54, with third loop controller 54 connected to control gain and phase adjuster 40.
Being in a simplified form for sake of illustration, the amplifier system 20 of FIG. 1 comprises three loops. A first loop of amplifier system 20 includes phase and gain adjuster 22, main power amplifier 26, coupler 28, attenuator 30, and subtractor 32. If the gain and phase shift through phase and gain adjuster 22, main power amplifier 26, and attenuator 30 equals the gain and phase shift through delay 36, an error signal indicative of the distortion of main power amplifier 26 is output by subtractor 32. But in order to equalize gain and phase shift through these paths, first loop controller 34 is used to produce control signals, applied on line 60, to phase and gain adjuster 22.
A second loop of amplifier system 20 comprises attenuator 30, subtractor 32, gain and phase adjuster 40, auxiliary amplifier 42, coupler 44, and delay 48. If the gain and phase shift through attenuator 30, subtractor 32, gain and phase adjuster 40, and auxiliary amplifier 42 equals the gain and phase shift through delay 48, except for a 180 degree phase shift, the distortion is added in opposite phase at coupler 44, thus canceling out the distortion of main power amplifier 26 on line 46. A third loop including attenuator 52 and third loop controller ensures phase and gain equality in these two paths.
Thus, the first loop described above with reference to amplifier system 20 creates an error signal which contains the intermodulation distortion from the main power amplifier 26. The second loop serves to cancel intermodulation distortion at output point 51, while leaving the carriers unaffected.
If the intermodulation distortion could be reduced in the main amplifier, the demands on the feedforward system would be lowered. For example, the error amplifier, e.g., auxiliary amplifier 42 in FIG. 1, must be dimensioned to handle the total intermodulation power. With less intermodulation power, a smaller error amplifier with a lower power consumption could be used, leading to higher efficiency of the overall amplifier system.
Accordingly, some amplifier systems which employ the feedforward technique also introduce a predistortion that is the reverse of the non-linearities of the main amplifier. FIG. 2 illustrates an example use of pre-distortion in a system such as that described above. The system of FIG. 2 basically involves addition of a pre-distortion circuit 80, as well as an additional phase and gain adjuster 22xe2x80x2. The additional phase and gain adjuster 22xe2x80x2 is added in series with the first gain and phase adjuster 22 (which can be a Cartesian gain and phase adjuster). The person skilled in the art knows how to construct and use a suitable pre-distortion circuit, for example with reference to U.S. Pat. No. 6,075,411 to Briffa et al., which is incorporated herein by reference in its entirety.
At high output levels, the gain of a power amplifier (such as power amplifier 26 in FIG. 2) may decrease due to compression. A purpose of predistortion is to add (complex) gain in signal path before the power amplifier in order to compensate for the decrease of gain in the power amplifier.
The pre-distortion gain and phase adjuster 22xe2x80x2 has the normalized transfer function 1+ip(x)+jqp(x), where ip(x)+jqp(x)=P(x) is the pre-distortion vector provided by pre-distortion circuit 80. The functions ip(x) and qp(x) are signal dependent and are in most cases polynomials. The combined output (seen at the output of pre-distortion gain and phase adjuster 22xe2x80x2) is provided by Expression 1.
y(t)=x(t) ((ic+jqc)(1+ip(x)+jqp(x)))xe2x80x83xe2x80x83(Expression 1)
It would, however, be convenient if the pre-distortion signal output by pre-distortion circuit 80 were added in the first Cartesian phase and gain adjuster 22, rather than in pre-distortion gain and phase adjuster 22xe2x80x2. FIG. 3 shows such an attempted implementation, with part of the input signal (obtained via coupler 82) and certain coefficients being used by pre-distortion circuit 80 to generate a pre-distortion vector P. In such case it would appear that the pre-distortion vector P is to be added by an adder 84 to the loop control vector C output by first loop controller 34, with the addition performed by adder 84 giving a composite control vector Comp which is applied to phase and gain adjuster 22.
In graphical form, the composite control vector Comp is illustrated by the right-most vector in FIG. 4. The corresponding output of gain and phase adjuster 22 is as indicated in Expression 2.
xe2x80x83y(t)=x(t)((ic+ip(x))+j(qc+qp(x)))xe2x80x83xe2x80x83(Expression 2)
As it turns out, however, the phase shift through phase and gain adjuster 22, main power amplifier 26, and attenuator 30 in the FIG. 3 implementation is typically about 10 to about 20 full 360 degree turns, and only small variations in power and amplifier characteristics produce considerable phase changes.
In the situation depicted in the first quadrant of FIG. 4, the total complex gain of the vector sum Comp (Comp is the vector sum of vector C and vector P) is higher than the vector C. But the vector C is prone to change, such change occurring when the phase of the signal path which includes the power amplifier changes (due, for example, to such factors as temperature changes). Since the first loop controller 34 adapts to these changes, the control vector C may rotate to an extent that it falls in another quadrant (for example as illustrated by the left-most vector C in the second quadrant in FIG. 4). The pre-distortion vector P will then have a wrong direction relative to C, with the result that pre-distortion vector P cannot be directly applied to phase and gain adjuster 22 under such circumstances. Rather, the vector P has to be changed to give the desired higher gain. An extreme situation occurs in the third quadrant wherein the vectors C and P have opposing directions, with the incorrect result that the vector sum Comp is actually shorter than vector C.
What is needed, therefore, and an object of the present invention, is a technique for rendering the pre-distortion vector P usable by a phase and gain adjuster of a feedforward amplifier system.
An amplifier system for radio frequency signals comprises a phase and gain adjuster which receives an input signal and a control vector for producing a distortion-adjusted input signal. The distortion-adjusted input signal is applied to a main amplifier which generates an amplified output signal. A control loop generates a control vector indicative of distortion of the main amplifier. A modified pre-distortion vector generator generates a modified pre-distortion vector which is combined with the control vector to produce a composite control vector. The composite control vector is applied to the phase and gain adjuster, thereby enabling the phase and gain adjuster to produce the distortion-adjusted input signal. In essence, the modified pre-distortion vector generator performs a vector multiplication (e.g., coordinate rotation) of a pre-distortion vector so that the resultant modified pre-distortion vector has a proper direction relative to the control vector.
In one example, non-limiting embodiment, the modified pre-distortion vector generator comprises both a pre-distortion circuit and a vector multiplier. The pre-distortion circuit receives the input signal and pre-distortion coefficients and produces a pre-distortion vector. The vector multiplier multiplies the pre-distortion vector output by the pre-distortion circuit and the control vector to generate the modified pre-distortion vector. An adder adds the modified pre-distortion vector and the control vector to produce the composite control vector. Advantageously, the composite control vector can be directly applied to the phase and gain adjuster.
An example specific implementation of the vector multiplier embodiment, the vector multiplier comprises plural multiplier elements, including a first multiplier, a second multiplier, a third multiplier, a fourth multiplier, a subtractor, and an adder. Given the fact that the control vector has components ic and qc and the pre-distortion vector has components i
and qp, the first multiplier multiplies ic and ip; the second multiplier multiplies qc and qp; the third multiplier multiplies ic and qp; and the fourth multiplier multiplies qc and qp. A product of the first multiplier and a product of the second multiplier are applied to the subtractor, whereas a product of the third multiplier and a product of the fourth multiplier are applied to the adder. Outputs of the subtractor and adder are applied to the adder which combines the modified pre-distortion vector and the control vector to produce the composite control vector.
In another example, non-limiting embodiment, the modified pre-distortion vector generator comprises a component vector multiplier and a pre-distortion circuit. The component vector multiplier multiplies pre-distortion coefficients by the control vector to produce modified pre-distortion coefficients. The pre-distortion circuit which receives the input signal and the modified pre-distortion coefficients to generate the modified pre-distortion vector. The modified pre-distortion vector is applied to the adder which combines the modified pre-distortion vector and the control vector to produce the composite control vector.
In the differing embodiment, the pre-distortion coefficients can be either adaptive coefficients or fixed coefficients as suits the particular application and/or environment of use.